The present invention relates to insulation material, particularly to aerogel type insulation, and more particularly to flexible aerogel composites for insulation (thermal and acoustic) and process of fabrication.
Insulators of various types have been developed for various application. For example, fiber glass, foamed organic polymers, and organic foam composites are well known and are used in the insulation, construction, and similar industries. These prior used foams, for example, generally are of relatively high density and were not suitable for many applications, such as high-energy physics applications, or as parts for inertial confinement fusion targets, which required very low densities.
In efforts to satisfy these needs, aerogels were developed. Aerogels are a unique class of ultra fine cell size, low density, open-cell foams. Aerogels have continuous porosity and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 angstroms. The microstructure of aerogels is responsible for their unusual acoustic, mechanical, optical, and thermal properties. These microstructures impart high surface areas to the aerogels, for example, from about 350 m.sup.2 /g to about 1000 m.sup.2 /g. Their ultra fine cell/pore size minimizes light scattering in the visible spectrum, and thus, aerogels can be prepared as transparent, porous solids. Further, the high porosity of aerogels makes them excellent insulators with their thermal conductivity being about 100 times lower than that of the prior known fully dense matrix foams. Still further, the aerogel skeleton provides for the low sound velocities observed in aerogels.
Currently, aerogels of various compositions are known, and these aerogels were generally referred to as inorganic (such as silicon aerogels) and organic (such as carbon aerogels). Inorganic aerogels, for example, silica, alumina, or zirconia aerogels, are traditionally made via the hydrolysis and condensation of metal alkoxides, such as tetramethoxy silane. Organic aerogels, such as carbon aerogels, are typically made from the sol-gel polymerization of resorcinol or melamine with formaldehyde under alkaline conditions. The aerogels may be pyrolyzed following the gel drying or extraction process.
Each type of aerogel, inorganic or organic, involve the formation of a gel, and drying of the gel by either air drying or supercritical extraction. The final composition of the aerogel is determined by the processing of the gel, which may produce an xerogel, an aerogel, or a hybrid xerogel/aerogel. Following the drying operation of the organic gels, for example, the aerogel may be pyrolyzed to produce a carbon aerogel.
One means, known as supercritical extraction, for removing water from the water-based gel or aquagel to form an organic aerogel, for example, is by extraction of the gel with a relatively lower surface tension fluid, such as carbon dioxide. Because water is immiscible with liquid CO.sub.2, the aquagels are first exchanged with an organic solvent such as acetone and then slowly dried inside a temperature-controlled pressure vessel. The critical point of carbon dioxide (T.sub.c =31.degree. C.; P.sub.c =7.4 MPa) is low enough to facilitate its removal without degrading the gel structure. The time required for supercritical drying depends on the thickness of the gel. Traditional aerogels are not flexible.
Both the inorganic and organic aerogels have common and unique characteristics, and these characteristics often limit the effectiveness of that type of aerogel for certain applications. While both types of aerogels may have similar cell/pore sizes, densities, and surface areas, they generally differ in atomic number (Z). For example, inorganic aerogels generally have a higher Z than organic aerogels, in that silicon has a Z of 14, aluminum has a Z of 13 and zirconium has a Z of 40, while organic aerogels, consisting mostly of carbon having a Z of 6, hydrogen a Z of 1 with some oxygen having a Z of 8. Both types (inorganic and organic) as very effective as insulators, as pointed out above, for both thermal and/or acoustic applications. However, since traditional aerogels are not flexible, they have been less effective in applications in aircraft, ice boxes, or refrigeration, or for use as energy absorbers, etc. The present invention provides a flexible aerogel fabrication process.